Living cells, including neurons, exhibit electrical potentials that, although quite small, can be measured. For example, the total charge contained in a typical cell is approximately 5×10−13 C, when measured for a voltage of −50 mV relative to the nutrient bath in which the cell is maintained. Obviously, obtaining such measurements can be difficult. Nevertheless, the effort may be worthwhile because a measurement of a cell's electrical potential contains information that can be very useful for evaluating the health and chemical composition of the cell. Such information may be particularly useful when successive measurements of a cell can be taken over extended periods of time. In addition, a cell's electrical potential may change in response to the exposure of the cell to a biological, chemical or pharmacological substance. Thus, the measurement of this change can be useful in detecting the presence of one of these substances.
Existing methods for measuring the electrical potential of a cell include procedures that require the insertion of a probe into the cell, or direct contact between the probe and the membrane of the cell. Fromherz et al., however, have proposed another approach for measuring the electrical potential of a cell which uses a custom-built field effect transistor (FET). This method relies on a capacitive coupling between the cell and the transistor that is on the order of picofarads. Consequently, the distance between the cell and the transistor must be around five one-hundredths of a micron (0.05 μm). As will be appreciated by the skilled artisan, this is a very small distance, and is virtually tantamount to actual contact.
In light of the above, it is an object of the present invention to provide a non-invasive system and method for measuring the electrical potential produced by a cell, in which the active electronics do not have to be in the immediate vicinity of the cell. Another object of the present invention is to provide a system and method for non-invasively measuring the electrical potential of the cell that is effective when the region of the probe that couples to the electric potential produced by the cell and passes the signal to the first stage electronics is as much as ten microns (10 μm) distant from the cell. Still another object of the present invention is to provide a system and method for measuring the electric potential produced by the cell within the nutrient bath that surrounds it, so that the cell can be maintained for extended periods of time in its required nutrient and electrolyte environment, without the cell being substantially disturbed. Yet another object of the present invention is to provide a system and method for measuring the electrical potential of a cell to determine whether the cell has been exposed to one or more biological, chemical or pharmacological substances. Another object of the present invention is to provide a system and method for measuring the electrical potential of a cell that is easy to use, is relatively simple to manufacture and is comparatively cost effective.